Transcript

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Earthquakes
Chapter 3 Earthquakes and Their Damages: Shaking Ground, Collapsing Buildings
Chapter 4 Earthquake Prediction and Tectonic Environment
Earthquakes are vibrations of the earth caused by the rupture and sudden movement of rocks that have been
strained (deformed) beyond their elastic limit. The forces that cause deformation and the build-up of strain
energy in the rock are referred to as stresses.
Earthquakes occur along faults. Faults are fractures in the lithosphere where regions of rock move past each
other (displaced)
• The focus is the point on the fault where rupture occurs and the location from which seismic waves are
released.
• The epicenter is the point on the earth’s surface directly above the focus.
• When the fault ruptures, waves of energy called seismic waves spread out in all directions.
Elastic Rebound Theory
Types of Faults
The majority of earthquakes (90%) are caused by rocks rupturing in response to tectonic stresses at active plate
margins.
Types of Tectonic Stress
Tensional Stress
Compressional Stress
Shear Stress
Faults can be divided depending on the direction of relative displacement.
There are 2 main categories.
 Dip-slip faults - where the displacement is vertical
 Strike-slip faults - where the displacement is horizontal.
Relative displacement is largely a function of the type of tectonic stress the rock is under.
Dip-Slip Faults - Normal Faults
• Normal faults result from tensional stresses along divergent boundaries.
• The hanging wall block moves down relative to the footwall block.
• Earthquakes generated tend to have low Richter magnitudes due to the tendency of rocks to break easily
under tensional stress.
• The earthquakes tend to be shallow focus (less than 20 km) because the lithosphere is relatively thin along
diverging plate boundaries.
• Examples - all mid-ocean ridges; Continental Rift Valleys such as the basin and range province of the Western
U.S. and the East African Rift Valley
Dip-Slip Faults - Reverse Faults
• Reverse faults result from compressional stresses along convergent boundaries.
• The hanging wall block has moved up relative to the footwall block.
• A Thrust Fault is a special case of a reverse fault where the dip of the fault is less than 15o
• There are two types of converging plate boundaries.
Subduction Boundaries
• At subduction boundaries cold oceanic lithosphere is pushed down into the mantle producing a continuum
of stress along the subducting plate. Shallow focus earthquakes can be generated near the trench, but
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focal depths can reach down to 700 km as earthquakes are generated along the subducting plate.
• Rocks are strong under compression and can store large amounts of strain energy before they rupture.
Therefore, these earthquakes can be very powerful.
– 1960 Southern Chili = 9.5
– 1964 Alaska = 9.2
Collision Boundaries
• At collision boundaries two plates of continental lithosphere collide resulting in fold-thrust mountain
belts.
• Earthquakes occur due to the thrust faulting and range in depth from shallow to about 200 km.
• Example: The Himalayas from the collision of India with Asia
Strike-Slip Faults - Transform Faults
• Strike-slip faults result from shear stresses acting on the lithosphere along transform boundaries.
• Horizontal motion can be right lateral or left lateral.
• Earthquakes along these boundaries tend to be shallow focus with depths usually less than about 100 km.
Richter magnitudes can be large.
• Example: The San Andreas Fault System
Intraplate Earthquakes
 These are earthquakes that occur in the stable portions of continents that are not near plate boundaries.
Many of them occur as a result of re-activation of ancient faults, although the causes of some intraplate
earthquakes are not well understood.
Earthquake Seismic Waves
Body waves travel through the interior (body) of the earth as they leave the focus. They include P-waves and
S-waves.
P - waves
• “Primary” waves
• Push-pull waves
S – waves
• “Secondary” waves
• Shear waves
Surface waves travel parallel to the earth’s surface. They are the slowest, involve the greatest ground motion,
and are therefore most damaging. They include Love and Rayleigh Waves.
Love Waves - complex, horizontal motion
Rayleigh Waves - Rolling or elliptical motion.
Earthquake Measurement
 Seismographs are sensitive instruments that detect and record ground shaking produced by earthquake
waves.
 Due to their different speeds, the different waves arrive at the seismograph at different times: first P-
waves arrive, then S-waves, then surface waves.
 Seismogram - the record of an earthquake as recorded by a seismograph. It is a plot of vibrations versus
time.
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Earthquake Seismic Waves
Wavelength – distance between two equal points on a wave.
Amplitude – the amount of positive or negative wave motion.
Period – the number of seconds between successive peaks of a wave. P = # seconds / 1 wave.
P and S waves have shorter periods (0.1 sec/wave to 1 sec/wave) than surface waves (1-3 sec/wave)
Frequency - the number of waves passing a point of reference per second. F = # waves / 1 second.
P and S waves have higher frequencies (1 wave/sec. to 10 waves/sec.) than surface waves (less than 1 wave/sec.)
Locating the Epicenter Via Seismograms
o P-waves are faster than S-waves, and the time gap between their arrival at a seismograph increases precisely
with distance from the focus. Basically, the lag time between the arrival of your first recorded P-wave and
first recorded S-wave is proportional to distance traveled.
o We can use the lag time between the P-waves and S- waves to calculate the distance to an earthquake! If we
do this for a minimum of three different seismic stations, we can precisely locate the epicenter. In the
figure, each circle has a radius equal to the distance to the earthquake from three separate seismic stations.
All three circles intersect at only one point -- the epicenter!
Earthquake Measurement
Richter Magnitude Scale
- ML; based on the highest amplitude wave measured on a seismogram, corrected for distance from the
seismograph to the epicenter
- ranges from 1.0 (smallest) to infinity, but 9.0 is typically the highest possible value for an earthquake.
- logarithmic scale: each whole unit on the Richter scale represents a ten-fold increase in wave amplitude (ground
shaking) and an ~ thirty fold increase in the energy released.
The Local Magnitude Scale developed by Richter was strictly valid only for certain frequency and distance
ranges. Therefore new magnitude scales were developed all calibrated to Richter's original method . These
include body-wave magnitude, MB, and surface-wave magnitude, MS. Each is valid for a particular frequency
range.
Moment Magnitude. Because of the limitations of all three magnitude scales, a new scale, known as moment
magnitude, or MW, was developed. MW is a measure of the seismic moment, or total energy expended during an
earthquake. MW depends on the rock strength, area of rock broken, and amount of offset across the fault.
Modified Mercalli scale
 Based on people’s reported perceptions of shaking (subjective), and the type and extent of damage produced
(objective).
 Ranges from I (not felt by people) to XII (catastrophic destruction)
Not all fault movements result in violent earthquakes. Some faults move slowly and fairly continuously, a
movement called fault creep.
Earthquake Hazards
earthquakes don’t kill people, buildings do”.
Earthquake Damage Susceptibility Depends on
 Magnitude of the earthquake - the higher the magnitude, the more intense the shaking, the longer the
duration of shaking, and the greater the displacement.
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 Distance from the epicenter – Seismic waves attenuate (amplitude diminishes) with distance.
 Surface Geology – Surface Faulting and ground rupture, and soil amplification.
 Integrity of Structures and Type of Construction - Building codes; Building material: concrete/masonry vs.
wood/steel. Structural Integrity.
 Integrity of Utilities.
 Population Density, building density, time of day, etc. - the more people and buildings, the greater the
potential for structural damage and death.
Earthquake Magnitude and Ground Acceleration
Ground acceleration is the rate of increase in velocity, or the strength of the shaking. During an earthquake, the
ground accelerates from being stationary to a maximum velocity before slowing and reversing its movement.
Acceleration is normally designated as some proportion of the acceleration due to gravity (g). 1.0 g is the
acceleration felt by a freely falling body.
Earthquake Magnitude and Shaking Time
An increase in magnitude significantly increases the time of shaking, and the potential damage
Magnitude and Fault Displacement
The amount of displacement during fault movement and the length of surface rupture are generally proportional
to the magnitude of the accompanying earthquake.
Distance from the Epicenter
Seismic waves attenuate with distance.
Surface Faulting and Ground Rupture - Land Uplift and Subsidence
Ground rupture and surface faulting occur due to horizontal or vertical displacement of faults that break the
surface. Areas right next to the fault can experience direct damage from the ground shifting upward (called
uplift) or downward (called subsidence).
Soil Amplification
Shaking is amplified as waves travel from solid bedrock to unconsolidated sediment to water-saturated sediment.
Liquefaction
Liquefaction is a quicksand like condition that occurs in water-saturated soil and rock. The shaking of earthquake
waves causes the soil or rock to turn into a weak, fluid-like mass. Structures built on areas that liquefy may fall
over or sink.
Landslides
Vibration of water saturated sediment can force water into pore spaces between sediment grains reducing
friction and permitting the mass to slide down slope.
Integrity of Structures and Type of Construction
 The Uniform Building Code provides a seismic zonation map that indicates the level of construction standards
for buildings
 Concrete and masonry structures are brittle and thus more susceptible to damage.
 Wood and steel structures are more flexible and thus less susceptible to damage.
 Most damage and collapse of structures occurs due to sideways movement of the ground from earthquake
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waves. This process is called horizontal ground acceleration, or base shear.
o Base shear causes the building to deform from a rectangle into a parallelogram.
o Base shear causes buildings constructed on so-called “cripples” -- short walls that raise the building
up from its foundation -- to fall sideways.
o The most deadly type of failure from base shear is “story-shift”, in which the sideways acceleration
causes floors to shift resulting in the collapse of either individual floors or the whole building -- a
situation called pancaking. Few or no occupants survive such collapses.
Mitigation of base shear includes:
o Shear walls: diagonal braces or plywood sheeting built into the walls, to keep the building from
deforming during base shear.
o Bolting to the foundation, so the building does not slide off.
o Base isolation: putting the building on large rubber pads, rolling wheels, or slippery Teflon plates. This
allows the ground to move under the building, thus isolating the building somewhat from ground
motion.
o Seismic joints: areas of flexible material, like rubber, form connections between different parts of a
building. These allow the separate areas of the building to shake independently.
 In addition to buildings, highway overpasses, bridges, and multi-decked freeways also suffer major damage
from base shear, most commonly due to the failure of the concrete supporting columns
New construction practices to reduce failure of highway overpasses include:
o Horizontal rebar wrapping added when casting concrete columns (left below).
o Retro fitting existing columns with steel jackets to make them stronger and more flexible (right).
Building Vibration and Oscillation
o Buildings have natural vibration frequencies in the same range as earthquake waves. Shaking of the building
will be amplified when the frequency of the building is close to the frequency of the waves produced by the
earthquake.
o Adjacent buildings of different heights will sway at different frequencies, and therefore can collide during
an earthquake.
Indoor Hazards
Aftershocks
Aftershocks are earthquakes that follow the largest shock of an earthquake sequence. The main earthquake
changes the stress pattern in areas around the epicenter, and the crust must adjust to these changes.
Aftershocks can continue over a period of weeks, months, or years. In general, the larger the main shock, the
larger and more numerous the aftershocks, and the longer they will continue.
Fires
Fires commonly break out during quakes due to ruptured gas lines or downed electrical lines. Impassible roads and
ruptured water mains compound the problem. In some urban quakes, fires have caused more damage than the
ground shaking itself.
The largest earthquakes do not necessarily kill the most people. Most of the high death counts come form
countries notable for poor building construction or unsuitable building sites.
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Earthquake Prediction / Forecasting
Based on knowledge of earthquakes and the tectonic environments that control them scientist can make
reasonable forecasts about where and how large an earthquake will occur at some future time.
However they have been less successful in finding ways to make short term predictions. Earthquakes appear to
be inherently unpredictable.
Nevertheless, great research effort has gone toward finding a reliable system for short term earthquake
prediction. One main avenue of research has been the study of earthquake precursors.
• Ground deformation: Measurements taken in the vicinity of active faults sometimes show that prior to an
earthquake the ground is uplifted or tilts due to the strain building on the fault.
• Foreshocks: Small earthquakes that precede a large quake by a few seconds to a few weeks. The pattern and
intensity of foreshocks usually increase in magnitude and may cluster or migrate down a fault to the place
where the main shock will eventually occur.
• Water Level in Wells: As rocks become strained in the vicinity of a fault, new fractures may form causing a
change in the path of groundwater subsequently causing changes in the water levels in wells.
• Emission of Radon Gas - Radon is an inert gas that is produced in the radioactive decay of uranium. Radon is
inert and remains in rock until some event forces it out. Deformation resulting from strain may form fracture
in the rock which could serve as pathways for the Radon to escape into groundwater resulting in background
radioactivity in wells.
• Abnormal Animal Behavior.
Alas, no reliable short-term precursors have been found. Therefore research today focuses on longer-term
warnings or forecasts. In this approach, geologists attempt to identify regions where large earthquakes are likely
to occur within the next several years or decades. While this does not provide short-term warnings, it is useful
for long-range planning for building codes and emergency response services.
Migrating Earthquakes
A few faults show a sequential migration of earthquakes along the fault with time believed to be the result of
progressive earthquake failure . For faults that follow this pattern it is believed that one earthquake triggers
the next. Ex. North Anatolian Fault in Turkey.
Seismic Gaps
A seismic gaps is a section of an active fault that has not had a recent earthquake. The theory here is that if a
portion of a fault has been “locked” for some time (i.e. has not had an earthquake in a long time), then strain may
have built up to especially high levels there, and a large quake may occur in the near future.
Earthquake Regularity
Recurrence interval is a statistical estimate of the expected time interval between an event of a given
magnitude. It is a statistical probability! The theory here is that faults should behave in the future like they
have behaved in the past, producing a characteristic number of quakes of particular sizes over a given time
interval.
Paleoseismology
Paleoseismology is the study of the long-term geologic history of faults to determine their earthquake history
and possible future activity. The methodology assumes that the earthquakes produced by the fault have a
characteristic recurrence interval.
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